1. Field of the Invention
The present invention relates to a laminate for flexible packaging and a process for preparing the same. More specifically, the invention relates to a laminate for flexible packaging using a reactive hot-melt adhesive agent and to a process for preparing the same.
2. Description of Prior Art
Foods have been packaged not only by using rigid containers made up of metals, glasses and plastic materials but also by using flexible materials. Flexible packaging has an advantage in that it is light weight and that the packaging container after used can be easily disposed of.
When used in the form of a laminate by sticking a variety of flexible blanks, the flexible packaging, despite of its thin film-like or sheet-like form, exhibits a variety of functions to a sufficient degree such as excellent gas barrier property, sealing reliability, strength and heat-sealability. In order to accomplish this object, there are used, in combination and as flexible blanks, a metal foil or a gas-barrier resin that imparts gas-barrier property against oxygen and the like, an olefin resin for imparting heat-sealability, and a biaxially oriented polyester film or a nylon film for imparting mechanical strength.
The laminate can be produced by using a variety of means such as dry lamination, sandwich lamination, extrusion coating and co-extrusion. In the case of a packaging laminate that is to be subjected to the high-temperature sterilization, good results will be obtained when a dry lamination is employed using an urethane-type or an epoxy-type adhesive agent.
Japanese Patent Publication No. 11912/1983 discloses an adhesive composition for a plastic-metal foil composite packaging material which contains a polyester glycol of a particular molecular weight, a polyesterpolyurethane polyol or a mixture thereof, a silane coupling agent and an organopolyisocyanate, and in which the equivalent ratio of an isocyanate group to active hydrogen lies within a predetermined range.
The urethane-type adhesive agent is applied in the form of an organic solvent solution onto the adhesion surfaces of the metal foil and the plastic film. During the step of adhesion, however, the solvent-type adhesive permits the organic solvent to vaporize and contaminates the working environment and the air. Moreover, the solvent partly remains in the layer of the adhesive agent and deteriorates the property for retaining flavor of the content.
A number of attempts have already been made using a solventless-type adhesive agent in order to solve the problem of environmental pollution caused by organic solvents. There have been known a variety of adhesive agents such as those of the urethane-type, epoxy-type and acryl-type. Even the former urethane-type adhesive agents can be classified into those of the polyether-type, polyester-type and the like depending upon the components that are used, i.e., classified into those of the single-component adhesives that are cured with the moisture and those of the two-component adhesives that comprise a main agent and a curing agent. All of them, however, exhibit fluidity that is necessary for being applied, and are cured by the action of a catalyst or a curing agent to finally accomplish the adhesion.
It was found that the widely known solventless-type adhesive agents give satisfactory results for many adhesion purposes but are not yet satisfactory for the purposes of flexible laminate for packaging. In producing the flexible laminate for packaging, the operation for winding the prepared laminate onto the roll is indispensable from the industrial point of view. During the winding operation, however, there arises a problem in that a cavity or a so-called tunneling at which the film substrate is not adhered develop in the direction of width of the laminate roll that is wound. Even when the tunneling does not develop during the winding, the next step is not executed right after the working of the laminate which is still uncured because of the reason that the tunneling develops during the unwinding.
The phenomenon of tunneling is caused by the cohesive-destruction of the adhesive agent due to the force that stems from a difference in the internal stress between the substrates that are adhered together. In the roll of the laminate, there inevitably develops a dimensional difference caused by an increase in the diameter between the film layer on the side of center of the roll and the film layer on the outer peripheral side of the roll, from which it will be understood that the tunneling phenomenon is not avoidable.
The conventional solventless-type adhesive agent has a problem of a very small initial adhesive strength as well as a problem of generally low coating operation efficiency. That is, since the adhesive agent has a considerably low cohesive force compared with the cohesive force of the film that is adhered, the adhesive agent must be applied maintaining a thickness as small as possible within a range in which it can be uniformly applied. Compared with the solvent-type adhesive agent, however, the solventless-type adhesive agent makes it very difficult to being uniformly applied maintaining a reduced thickness.